1. Field of the Invention
The present invention relates to a cylindrical vibration isolating device suitable for a sub-frame mount or the like.
2. Description of the Related Art
As an example of cylindrical vibration isolating devices there is known a sub-frame mount. This sub-frame mount has an outer cylinder which is split right and left by a slit, and, when being mounted on a sub-frame, the slit is narrowed to reduce the diameter of the outer cylinder so as to compress an elastic body between an inner cylinder and the outer cylinder. There is also known such a sub-frame mount that projecting portions which project in the radial direction each from the outer cylinder to the inner cylinder and from the inner cylinder to the outer cylinder are provided spaced apart in the axial direction of the inner cylinder. When being mounted, the elastic body is compressed by each of tips of the projecting portions to increase the spring constant in the direction orthogonal to the inner cylinder axis while the spring constant in the axial direction is increased by the shearing action of the elastic body interposed between the couple of projecting portions.
Patent reference 1: Japanese patent laid-open publication No. H01-126447.
Since the axial direction of the inner cylinder corresponds to the input direction of the principal load, the spring constant in this direction is required to be increased, and the spring constant in this direction is influenced by the volume of the elastic body between the couple of projecting portions spaced apart in the axial direction of the inner cylinder. For increasing this volume the space between the tips of the projecting portions needs to be narrowed, but, if it is narrowed too much, the spring constant in the axially perpendicular direction is increased excessively. Accordingly, while this space must be increased to a certain extent, in that case, the volume of the elastic body between the projecting portions is decreased so that the spring constant in the axial direction is decreased below the requested level. Then, since the displacement in the axial direction is not able to be controlled only by the spring in this part, it is necessary to control the displacement by having a stopper 4A, as shown in FIG. 3 in phantom line, projected axially outwardly from the surface of the elastic body. However, since such stopper 4A increases the size in the axial direction of the device, there may be cases where the layout becomes difficult, depending on the models.
Further, when the stopper 4A exists, the stopper 4A itself has a large value of spring constant, and, when the stopper 4A is in contact with an engine side at the time of idling operation, the micro-vibration may be transmitted to the vehicle body side, so that it is required to isolate such vibration.
The present invention therefore aims to provide the vibration isolating device capable of forming such a large spring constant that the above mentioned stopper can be dispensed with, also capable of increasing the degree of freedom to control the ratio between the spring constant in the axial direction and the spring in the axially perpendicular direction, and capable of more improving the vibration isolating performance.